1. Field of the Invention
This invention relates to a process for producing a preform for a chalcogenide glass fiber of a core-cladding structure, a process for producing a chalcogenide glass fiber suitable to the transmission of infrared rays, particularly rays of 1 to 14 .mu.m by using the above preform, and a chalcogenide glass fiber of a core-cladding structure excellent in infrared ray-transmissibility, particularly suitable to transmission of infrared signal light.
2. Prior Art
A cast method has been known as a method for producing a preform of a core-cladding unitary structure using chalcogenide glass. This cast method comprises casting molten core glass into a tube of cladding glass.
As other methods, JP-A-1-230,440 proposes a method for producing a preform of a core-cladding unitary structure by inserting a cladding glass tube into a quartz tube closed at its bottom, further inserting a core glass rod into the cladding glass tube, and heating the outer side of the quartz tube of the resulting assembly in this state by means of a ring heater while reducing the pressure in the space between the core glass rod and the cladding glass tube and applying a pressure to the space between the cladding glass tube and the quartz tube, thereby uniting the core glass and the cladding glass.
Chalcogenide glass fibers can be obtained by heat and drawing the preform obtained by the above cast method or the method of JP-A-1-230,440.
Moreover, there has been also known a method for producing a chalcogenide glass fiber directly from a core glass rod and a cladding glass tube without preparing a preform. As this method, JP-A-1-226,748 proposes a method for producing a glass fiber by inserting a cladding glass tube having contained therein a core glass rod, into a quartz tube having at its bottom a nozzle having an aperture smaller than the outer diameter of the cladding glass tube and drawing the resulting assembly while locally heating the lower portion of the assembly than the lower end of the quartz tube while controlling the gas pressure in the space between the cladding glass tube and the quartz tube so as to become higher than the gas pressure in the space between the core glass rod and the cladding glass tube.
It is well known that chalcogenide glass is thermally unstable. Accordingly, in the method for producing a chalcogenide glass fiber by the above-mentioned cast method, there is such a problem that the glass is devitrified in the course of the production of a preform. In addition, the molten core glass easily takes in bubbles during the casting and these bubbles remain as broths at the interface of the core glass and the cladding glass. When the preform containing these broths is formed into fibers, the broths have become a cause of increasing the transmission loss. By this cast method, it has been impossible to produce a preform for a single mode fiber in which the core diameter is much smaller than the cladding diameter.
The method stated in JP-A-1-230,440 for obtaining a preform by uniting the core glass and the cladding glass under control of the pressure in a quartz tube whose bottom is closed can considerably inhibit the devitrification of glass and the generation of bubbles in the core glass or at the core glass-cladding glass interface as compared with the cast method. However, the preform obtained cannot be said to be sufficient in adhesion between the core glass and the cladding glass, and an improvement in adhesion has been desired. Also, this method for producing a preform was unable to produce a preform for a single mode fiber in which the core diameter is much smaller than the cladding diameter.
On the other hand, in the method stated in JP-A-1-226,748 for producing a glass fiber directly from a core glass rod and a cladding glass tube without preparing any preform, there has been such a problem that since the cladding glass tube having contained therein the core glass rod must be heated at a high drawing temperature from the beginning, the chalcogenide glass tends to be devitrified and the composition tends to be changed by volatilization. The said method for the direct production of a fiber has such a disadvantage that it is difficult to obtain a single mode fiber in which the core diameter is much smaller than the cladding diameter. Moreover, according to the method for the direct production of a fiber, when it is intended to produce fibers having different diameters, a plurality of quartz tubes having drawing nozzles having correspondingly different diameters must be used and when a fiber having a specific diameter has become necessary, it is difficult to obtain such a fiber in a short period of time.
Furthermore, as to the chalcogenide glass fiber of a core-cladding structure, the technique stated in, for example, JP-A-3-8,742 has heretofore been known. This technique intends to provide a power transmission fiber in which each of the core glass and the cladding glass is composed of three elements of As (arsenic), S (sulfur) and Ge (germanium) for enhancing particularly the heat resistance and Se (selenium) is substituted for a part of the S (sulfur) in the core glass for controlling the numerical aperture (NA) of the fiber.
However, in the case of the above technique, in order to enhance the heat resistance of a power transmission infrared fiber, Ge element is contained in both the core glass and the cladding glass, and hence, the fact that both contain Ge makes it basically difficult to make the transmission loss lower than a certain level. Also, since Ge is contained in both the core glass and the cladding glass, a sufficient difference in thermal expansion between the two is not obtained in the formation of a fiber and hence it is impossible to enhance the mechanical strength of the fiber.
In addition, in the case of the above core-cladding structure, there is such a problem that infrared rays passing through the cladding side become a noise when an optical signal is transmitted and the optical signal to be transmitted in the core undergoes a disturbance.